Elasmobranch fishes possess a sensory system specifically adapted for responding to the weak electric fields that exist in all aquatic environments. Electroreceptive information can be used in prey capture as well as in orientation and navigation. Recent evidence indicates that an electrosensory system homologous with this elasmobranch system was present in the primitive ancestor of all living vertebrates and later was lost when vertebrates moved onto land. The aim of this research is to understand the functional organization of medullary and midbrain centers for electrorectption in elasmobranchs. Physiological techniques for studying single neuron activity and averaged evoked potential responses are being used to discover what features of the electrosensory world are extracted and emphasized and what features are filtered out by the central nervous system. In addition experimental anatomical techniques utilizing Horseradish peroxidase are being utilized to chart the distribution of peripheral electroreceptor afferents in the medullary electroreceptor nucleus as an aid to understanding functional organization. Studies of electroreceptive responses at more than one level in the brain should provide insight into the sequential and parallel processing of information in this ancestral system and will have implications for CNS processing of sensory information by vertebrates in general.